U.S. Pat. No. 6,401,056 B1 describes the recording of information about punching tools during the lifetime of a tool. A record is made of regrinding operations performed on dies and of a changing size of a height of the die that is associated with regrinding. The recorded tool data are intended, in particular, to enable the tool user to determine when a punching tool needs to be repaired or replaced.
The dies normally used in practice are dies having a die orifice whose wall extends at a so-called “relief angle” with respect to the wall of the punch entering the die orifice. The relief angle is intended to ensure that the material which is punched out can reliably leave the die and that the punch can return to its home position without hindrance following penetration of the workpiece to be processed and following associated entry into the die orifice of the die. To obtain the highest possible process reliability during punching, the relief angle on dies would have to be as large as possible.
Regrinding of dies is typically carried out because of wear to the cutting edge of the die and involves removal of material from the die in the entry direction of the punch. Owing to the relief angle, the removal of material is accompanied by enlargement of the cross-section of the die orifice at the level of the cutting edge of the die (“die cutting edge cross-section”). At the same time, the die cutting edge cross-section is a determining factor for assignment of the die to a punch cooperating therewith (“punch assignment”) and/or for the thickness of the workpiece that can be processed by means of the die and the punch assigned thereto (“workpiece thickness assignment”). For example, in the case of a punching tool set comprising a punch of circular cross-section and a die with a die orifice of circular cross-section, ideally a cutting gap of a width dependent on the thickness of the material to be processed remains between the cutting edge of the die and the punch. If, for example, a circular cut-out of 5 mm diameter is to be made in a metal sheet of 2 mm thickness, there is provided for that purpose a punching tool set comprising a round punch with a diameter of 5 mm and a die with a circular die orifice, the cutting edge of which die has a diameter of 5.2 mm.
For individual processing tasks, for example for a specific thickness of metal sheet and for a specific punching diameter, users of punching tools keep a stock of punching tool sets specifically designed for the particular processing task. Regrinding of the die of such a punching tool set, for example because of wear to the cutting edge of the die, is envisaged only on a scale such that the enlargement of the die cutting edge cross-section, which is associated with regrinding of the die, does not result in an unduly wide cutting gap being produced between the cutting edge of the die and the assigned punch.
Owing to the relief angle of the die, however, the die cutting edge cross-section is typically widened, even as a result of a slight regrinding of the die, to a value that should not be exceeded for a given punch if high-quality processing results are to be achieved. In the case of an unduly wide cutting gap, only processing results of an inferior quality would be achievable using the punching tool set. Once the size of the die cutting edge cross-section reaches its limit value, further regrinding is usually abandoned and the die is instead removed from stock and scrapped.